This invention relates generally to a quartz analog movement for a timepiece of the type utilizing a Lavet stepping motor, and more particularly it relates to a movement for a three-hand quartz analog wristwatch with a Lavet stepping motor and an improved arrangement in the speed reducing gear train driving the hands which, in turn, permits the use of a larger size energy cell in the movement.
A Lavet type stepping motor periodically steps a small permanent magnet rotor with a driving pinion through 180.degree. and a speed reducing gear train of gears and pinions is arranged in the movement frame to drive the hands of the timepiece, which are generally mounted upon sleeves or spindles rotatably mounted in the center of the watch dial. It is desirable to use as large an energy cell as can be fitted into the movement in the space not occupied by these gear members, so that the energy cell lasts as long as possible before it is necessary to replace it. Since the capacity of the energy cell is generally related to its volume of active material, its size can be increased either by increasing its thickness or its diameter, which interfere with the very limited space in a wristwatch movement for the gear train members, the stepping motor, the integrated circuit, and other components necessary to functioning of the quart analog movement.
One proposal for accommodating the necessary speed reduction between the stepping motor rotor and the central hand spindle has been to drive a minute wheel directly at its periphery using a relatively large wheel which overlaps a relatively thin but large diameter energy cell. This arrangement is shown and described in assignee's U.S. Pat. No. 4,647,218 issued Mar. 3, 1987 in the name of Paul Wuthrich.
Another known arrangement for reducing the speed of the stepping motor rotor to drive a "seconds" wheel attached to a "seconds" hand spindle is through the use of an intermediate wheel assembly with a gear and pinion disposed between the rotor of the stepping motor and the central assembly of spindles and sleeves driving the hands of the timepiece. Several such arrangements are disclosed in published U.K. patent application No. GB 2 121 991 A filed Apr. 8, 1983.
A problem arises in the design of the gearing, since if the largest possible energy cell is employed, its outer diameter lies very close to the movement's center. The design of the gearing, especially of the seconds wheel and the intermediate wheel arrangement must be adapted to the remaining space. One known solution as in the aforesaid U.S. Pat. No. 4,647,218 is to use a thin energy cell and to have the battery overlapping a large diameter second wheel. This provides sufficient battery life if the stepping motor is indexed only once per minute, but if the stepping motor is indexed once per second in a three hand watch, a thicker energy cell would provide more capacity.
Another proposal is to use a small diameter energy cell in order to allow the diameter of the second wheel to be placed adjacent and in line with the energy cell. This is undesirable, since by increasing the diameter of the energy cell, more capacity could be achieved.
A third known proposal is to employ a small diameter "seconds" wheel beside an energy cell which is large in both thickness and diameter. However, in this known proposal, the intermediate wheel assembly performing the gear reduction overlaps the axis of the "seconds" wheel so that a special intermediate bridge has to be employed in order to journal or rotatably support the seconds wheel.
A fourth known proposal, as described in U.S. Pat. No. 4,518,884 issued May 21, 1985, is to place a gear train arbor of ferro-magnetic material, e.g. steel in a deep groove in one of the stator pole pieces. However, this requires compensation for the presence of the arbor by shifting the pole pieces, and requires location of the groove at 90.degree. with respect to NS of the magnet poles when the rotor is in the rest position.
Accordingly, one object of the present invention is to provide an improved arrangement for the reduction gearing in a timepiece movement which permits a larger energy cell to be employed in the timepiece.
Another object of the invention is to provide an improved gear reduction assembly for a three hand quartz analog timepiece with a Lavet stepping motor which permits a larger energy cell.
Still another object of the invention is to provide an improved gear arrangement for mounting an intermediate wheel assembly between the rotor of a Lavet stepping motor and the "seconds" wheel driving the second hand at the center of the timepiece movement.